Quantum technology is on the verge of becoming the next transformative discipline. Quantum entangled sources, i.e. sources that emit entangled particles, are a core quantum technology. Entangled sources have one property that no classical source has, that is quantum entanglement. Once described by Einstein as “spooky action at a distance”, entanglement is now an extremely powerful resource for quantum computing, quantum communication and quantum sensing. Entangled photons are pairs of photons exhibiting quantum correlations which can be harnessed to perform more efficient computation, higher resolution imaging, or secure cryptographic key distribution. Different types of entanglement exist, such as position-time entanglement, quadrature entanglement, frequency entanglement and modal entanglement. Polarization-entanglement is by far the most widely encountered and arguably the most promising for practical applications, because polarization correlation is easy to measure and verify, without requiring stable interferometers. Moreover, telecom-wavelength entangled photon sources are particularly useful, because their wavelengths correspond to the low-loss region of the existing telecom fiber, enabling easy distribution of entanglement over long distances, which is essential to distributed quantum computing and quantum cryptography.
While there are a number of technologies that can produce polarization-entangled photon pairs in the telecom wavelength region, they tend to be bulky and complex. The quantum industry cannot survive on fragile, bulky and expensive entangled sources that work only in highly controlled laboratory environments. The more robust, compact, efficient, and affordable the entangled sources become, the sooner quantum technology will flourish.
Polarization-entangled EPR (Einstein-Podolsky-Rosen) pairs can be generated through spontaneous parametric down conversion (SPDC) in nonlinear optical media, typically birefringent crystals or waveguides that have a large second-order nonlinearity, χ(2). Pump photons traveling in the nonlinear medium have a small probability of decaying into two correlated daughter photons (the signal and the idler). Momentum conservation (i.e., phase matching) is satisfied either through choosing a particular crystal orientation, or through quasi-phase matching (QPM), attainable with a periodic structure. Polarization entanglement of the two daughter photons is either inherent in the SPDC process or can be achieved through post selection or erasure of distinguishability.
While crystals and waveguides with large χ(2) can yield high-flux photon pairs, the coupling efficiency into a fibre is poor. They also require bulky optical elements and precision alignment. Fibre-based photon pair sources in the telecom wavelength region are desirable as generation and transport of photon pairs can then be done seamlessly in an all-fiber system, eliminating bulky optics and alignment.
However, conventional fibres do not exhibit 2nd-order nonlinearity (χ(2)=0) due to glass symmetry. Prior work on fiber-based polarization-entangled photon pairs had been limited to using third-order nonlinearity, χ(3), via the spontaneous four-wave-mixing (SFWM) process, involving two pump photons having nearly the same wavelength (near degeneracy) as those of the signal and the idler in the telecom band.
Major disadvantages of the SFWM approach are:
(1) Due to the weak χ(3) in fibre, typically very long fibres are required (e.g., 300 m of DSF); in some specialty fibers (such has photonics crystal fibers), high nonlinearity can be achieved, but the fiber fabrication process is complex and such photonic crystal fibers are expensive.
(2) Operating near degeneracy requires the suppression of Raman-scattered photons as they spectrally overlap with the signal and the idler. Raman suppression is achieved by cooling the fibre in either liquid nitrogen or liquid helium, but residual Raman noise is still a major limitation to the entanglement quality of the source;
(3) The χ(3) tensor of silica is such that the photon pairs have the same polarization as the pump, and therefore are not polarization-entangled. Additional steps are required to convert them into polarization-entangled pairs, adding more complexity and cost.
These disadvantages largely diminished the attractiveness of using fibre, as the overall system would still be bulky and costly.
A further observation is that in the majority of the experimentally demonstrated systems that produce polarization-entangled photon pairs, the birefringence of the nonlinear medium (be it a crystal, a waveguide, or a polarization-maintaining fiber) is very large, leading to distinguishability due to polarization-dependent differential group delay (DGD). This DGD must be compensated to obtain high quality polarization entanglement, introducing an additional degree of complexity.
Although progress has been made in the design of bulky nonlinear crystals or waveguides, challenges remain. Operation and delicate alignment in free-space and bulk optics is challenging for infield applications. In addition, due to the large group birefringence in nonlinear crystals, temporal compensation is always needed to achieve polarization entanglement. One has to use interferometer, or an additional birefringent crystal, or a pre-compensator for pump, to eliminate the temporal distinguishability in the birefringent biphoton source. Such requirements increase the complexity and reduce the robustness of an entangled photon source.
An initial proposal of using periodically poled fiber for polarization-entangled photon generation was published in L. G. Helt et al., Proposal for in-fiber generation of telecom-band polarization-entangled photon pairs using a periodically poled fiber, Optics Letter, 34, Issue 14, pp. 2138-2140, 2009.
In 2012, the direct entangle generation was experimentally demonstrated using a fibre without any compensation or erasure of distinguishability; see for example Zhu, Eric Y., et al. “Direct generation of polarization-entangled photon pairs in a poled fiber.” Physical review letters 108.21 (2012): 213902.
The following article discusses broadband emission of the source: E. Y. Zhu, et al, “Poled-fiber source of broadband polarization-entangled photon pairs,” Opt. Lett. 38, pp. 4397-4400, 2013.
The following article discusses compensation-free broadband polarization entanglement generation achieved in a periodically poled fiber: Chen, Changjia et al., Compensation free broadband entangled photon pair sources, Optics Express 25, pp. 22667-22678, Sep. 8, 2017. This reference is incorporated herein by reference in its entirety.